One field of application for the invention is holding together a bone and a support member in a desired relative position (while allowing in some cases a limited amount of relative movement), for example to aid in healing of breaks or to correct bony structure deficiencies and abnormalities. In particular, sufferers of abnormal spine curvature or other spine deficiencies may benefit from the invention.
The spine is formed of superposed vertebrae, normally aligned along a vertebral axis, from the lumbar vertebrae to the cervical vertebrae, each having an anterior part: the vertebral body, and a posterior part: the vertebral arch (or neural arch), the anterior and posterior part enclosing the vertebral foramen. Each vertebral arch is formed by a pair of pedicles and a pair of laminae, with transverse processes and/or a spinous process (or neural spine) projecting therefrom. The transverse processes and the spinous process project opposite to the vertebral foramen.
When the vertebrae are articulated with each other, the vertebral bodies form a strong pillar for the support of the head and trunk. In between every pair of vertebral bodies, there is an intervertebral disc.
When the spine of a person has abnormal curvature or other deficiencies (e.g. a damaged intervertebral disc), the vertebrae are typically too close together or spaced too far apart, and there is a need to stabilize the vertebrae in a correct position relative to one another. Mainly, there is either a need to compress the vertebrae (i.e. to bring and hold them closer together) or a need to distract the vertebrae (i.e. to space and keep them away from each other). In order to do this, various kinds of stabilization devices known in the art may be used.
Typically, known stabilization devices include at least two bone anchors configured to be fastened, respectively, to at least two distinct vertebrae, and a rod for connecting the anchors together, thereby providing stabilization between the vertebrae.
In certain known stabilization devices said anchors are hooks that rest on the vertebral laminas and go along the internal wall of the vertebral foramen. Examples of conventional hooks are disclosed, for instance, in U.S. Pat. No. 4,269,178, in published PCT application no 2005/023126 or in published US patent application no 2007/0161990 A1.
A hook has the advantage of providing a rigid and strong anchoring to the vertebra but, in certain cases, during implantation, the physician (or other operative) may have difficulty in placing the hook correctly on the vertebra because the hook does not fit well or slips on the vertebra. Moreover, in certain cases, after implantation of the whole stabilization device, there is a risk that the hook will slip on (or even disengage from) the vertebra, thus reducing or canceling the intended stabilization effect. Sometimes, this risk leads the physician to use more hooks than theoretically needed.